ABSTRACT Immune stimulatory pathways are dysregulated during chronic viral infections, preventing timely T cell and antibody mediated immunity. Human persistent viruses, including human immunodeficiency virus (HIV), hepatitis C and B viruses (HCV and HBV) cause tremendous disease burden worldwide but are restricted to human and nonhuman primates, which in turn poses great limitations for experimental-based research. Importantly, lymphocytic choriomeningitis virus (LCMV) in its natural rodent host has been successfully used as a model system to uncover common principles underlying the regulation of immune responses during persistent viral infections, often highlighting basic mechanisms that were later extended into humans. We have found that interleukin-6 (IL-6) and IL-27 are essential for optimal CD4 T cell differentiation and CD4 T cell survival, respectively, while they redundantly induce IL-21, a critical factor for CD8 T cell and antibody responses, late after infection. Consistently, both IL-6 and IL-27 were absolutely required to control chronic (but not acute) LCMV infection. More recent data indicate that late IL-27 derived from B cells is crucial for viral control and it is regulated by host microbiota during chronic LCMV infection. The overall goal of the current proposal is to fully dissect the mechanisms underlying the antiviral effects of B-cell-derived IL-27 as well as its regulation by host microbiota and related metabolites during persistent infection. To accomplish this goal, we propose three specific aims. In Aim 1 we will investigate the relationship between B-cell-derived IL-27 and antiviral responses during chronic LCMV infection. We will test the hypothesis that B cell-derived-IL-27 is both a dependent and a driver of CD4 T cell responses during chronic infection, and indirectly enhances antibodies and CD8 T cells, promoting viral control late after infection. In Aim 2 we will identify the microbiome commensals that enhance IL-27 production by B cells and the impact for antiviral responses and viral control during chronic LCMV infection. We will investigate the possibility that Segmented Filamentous Bacteria (SFB), a commensal that associates with accelerated viral control, enhances IL-27 production by B cells in small intestine as well as T cell responses both locally and at distal sites, overriding the need for IL-6. Finally, in Aim 3 we will identify microbiota-related metabolites that modulate IL-27 production by B cells and the impact for antiviral responses and viral control during chronic LCMV infection. We will investigate the metabolites that associate with the microbiota driving IL-27 production by B cells and evaluate their effects on IL-27 expression (in mouse and human B cells) as well as T cell responses and viral control in LCMV chronically infected mice. The knowledge gained from this study will not only enhance our understanding of the basic biology of key immune regulators (i.e. IL-27, microbiome and related metabolites) but may also be valuable for therapeutically manipulating these factors during infections and perhaps other immune diseases.